The -I and +Q elements correspond to R’G’B’ values of [-0.3824, 0.1088, 0.4427] and [0.2483, -0.2589, 0.6817], respectively; and to 10-bit 601Y’CBCR values [0, 228, -244] and [0, 345, 159]. To produce RGB-legal codes having the same hue and saturation as -I and +Q, and having minimum luma, use R’G’B’ values [0, 0.2456, 0.412545] and [0.253605, 0, 0.470286], respectively. See SMPTE RP 219.

SMPTE RP 219, High-Definition,

Standard-Definition Compatible

Color Bar Signal.

-I, +Q, and pluge elements in SD colourbars

The lower-left quadrant of the SMPTE colourbar pattern contains subcarrier frequency components at -I and +Q phase. These elements were designed to exercise the encoding and decoding axes of the original NTSC chroma modulation method (circa 1953). Encoding and decoding on I and Q axes fell into disuse around 1970, being replaced by encoding and decoding on the B’-Y’ and R’-Y’ axes, so the utility of this portion of the signal is now lost. The historical -I and +Q elements contain high chroma resting upon black. These combinations correspond to illegal mixtures of R’G’B’ where one component is dramatically negative; consequently, the -I and +Q elements are not representable in the positive R’G’B’ domain.

The -I element, the +Q element, and the negativegoing element of pluge are generated synthetically. None of these elements represents picture information; none is useful in evaluating pictures; and none can be generated in – or survive transit through – the positive R’G’B’ domain between 0 and 1 (or 0 and 255, or even 16 through 235 or 64 through 940 if footroom is clipped).

In fact, it is not just the -I, +Q, and pluge elements of colourbars that are synthetic: The entire signal is generated synthetically! We call it the colourbar signal, not the colourbar image, because the -I, +Q, and pluge elements cannot be represented in nonnegative RGB components. The colourbar signal represents values of R’G’B’ as if they came from a gamma-corrected camera and were inserted prior to an encoder. R’G’B’ values of colourbars are implicitly gamma-corrected.

HD colourbars

Figure 36.1 overleaf is a sketch of an image produced by the SMPTE colourbar test pattern for HD.